Recently, demand for water of high purity in industrial applications has been markedly expanding in both the quantity and quality thereof. In particular, production of integrated circuits requires water of extraordinarily high purity, typically for rinsing semiconductor wafers. Such water of extra high purity is termed ultra pure water which consists only of water with little or no other substances or contaminants. Another important application of ultra pure water is found in the pharmaceutical industry as a charge for preparation and as wash water for final washing steps, for example.
Various processes have hitherto been proposed and practically used for the preparation of ultra pure water. Typical processes include reverse osmosis, adsorption on activated carbon, adsorption on ion exchange resin, ultrafiltration and "UV oxidation" so-called, which are adopted separately or usually in any appropriate combination thereof for a particular case and which are in any case combined with one or more previous treatment steps consisting essentially of one or more mechanical filtration steps, such as those by flocculation or coagulation, by solid contact sedimentation, by sand filter, by filter press and by other conventional filter means, to be appropriately selected dependent upon the nature of raw water to be treated, the nature of contaminants in the raw water and the amount of the contaminants and others. Thus, the preparation of ultra pure water has always relied on a multi-step process for purification of water.
One of most typical processes established and practiced in the prior art for the preparation of ultra pure water from a raw water such as city water, well water and industrial water comprises the steps of prefiltration of the raw water through one or more mechanical filtration means for the removal of solid particulate contaminants having particle sizes of about 5-10 .mu.m and larger; reverse osmosis for the removal of at least 99% of solid particulate contaminants, at least about 90% of ionic contaminants and at most about 80% of organic contaminants included in TOC contents; sterilization with UV radiation for destroying microorganisms contained in the water; ion exchange treatment for the removal of ionic contaminants to a level being substantially complete; and ultrafiltration for the removal of residual trace contaminants to yield an ultra pure water desired. In such a multi-step process, it is of course customary in the art to make some change of the sequence of steps or replacement, addition or other modification of steps, depending upon the nature and quality of raw water to be treated and upon the quality of ultra pure water desired. For example, a "UV oxidation" step, if desired to be incorporated into the above-mentioned typical multi-step process, will usually and advantageously be inserted after the prefiltration step.
According to the processes already proposed for the preparation of ultra pure water, ionic impurities could be removed to a level lower than the minimum detection level of the measuring instrument used, but complete removal of TOC was difficult and still impossible. Thus, the TOC value of ultra pure water produced by the prior art processes was usually about 0.1 mg C/l at most, provided that the value was steadily achievable for a reasonable period of continuous operation. Even if the TOC value of lower than 0.1 .mu.mg C /l were attained, it would be instable and would make the quality of ultra pure water fluctuable due to changes in the nature and concentration of organic impurities included in TOC content, thus posing a serious problem to be solved by further improvements.
As already mentioned, the quality requirement in TOC content for ultra pure water, particularly to be directed to the rinsing of semiconductor wafers called electronic grade water by ASTM is becoming severer year after year. Thus, in most recent years, according to proposed ASTM Standard for electronic grade water, Type E-I (the highest grade) and Type E-II (the next high grade) of such ultra pure water are required to satisfy the TOC of 50 ppb (0.05 mg C/l and 200 ppb (0.2 mg C/l, respectively. From this particular point of view, that is in respect of TOC content of ultra pure water required, typical prior art processes for the purification of water as above-referred to may be evaluated as summarized below.
Generally speaking, processes relying on either membrane-filtration or adsorption technique intend to remove simultaneously both the impurities included in TOC and those not included in TOC. Since, however, the higher the value of TOC in the water to be treated, the higher the load added to the apparatus to be used for the purification of water, there will occur, in the membrane-filtration processes, lowering of the output of purified water due to clogging of membrane and also lowering of the efficiency of the removal of impurities due to deterioration of membrane and, in the adsorption processes, saturated adsorption leading to leakage of impurities because of difficulty in control of amount of saturation adsorption.
In the membrane-filtration processes, the removal of organic impurities is usually effected by reverse osmosis where the organic impurities are removed together with ions and fine particulate materials after some previous treatments by mechanical filtration methods appropriately selected are taken, but the removal efficiency achievable thereby is within the range of 30-80% at the maximum depending upon the concentration of organic impurities in the water to be treated. On the other hand, the ultrafiltration processes are solely directed, as known, to the removal of organic impurities of high molecular weights, thus having been used mainly for the purpose of removal of fine particulate materials rather than the removal of TOC contents. As is well-known, organic impurities of low molecular weights contained in water can almost never be removed by ultrafiltration.
Adsorption processes such as those with ion exchange resins and those with activated carbon have such a low level of capability of TOC-removal as 50% at most depending upon the concentration of TOC in water to be treated. They also have such disadvantages that their applications are limited because it is impossible for ion exchange resin adsorption to remove any organic impurities with no electric charge and it is impossible for activated carbon adsorption to adsorb any organic impurities of low molecular weight, with a further possibility that ionic impurities are eluted, and that the control of adsorption saturation is difficult for all the processes in question.
"UV oxidation" so-called which has recently been developed mainly for the purpose of reclamation of waste wash water used for rinsing in various steps for the production of integrated circuits summarily comprises irradiating with UV light such waste wash water to be reclaimed to which hydrogen peroxide has been added, thereby causing oxidative decomposition of mainly TOC contents such as methanol, isopropyl alcohol, acetic acid, surfactants, photoresist, etc. However, UV oxidation is still not satisfactory in that a relatively long time is required, for example 2-4 hours when UV light is irradiated at an intensity of 2-10 KW/m.sup.3 of the water to be treated, and the capability of decomposition by this method is limited to about 1/2-1/5 of the initial TOC content at the maximum. In this connection, it is to be added by way of reference that a simple UV irradiation which is a well-known step to be adopted during the preparation of ultra pure water is for the purpose of sterilization or destruction of microorganisms as, for example, disclosed in T. L. Faylor et al's U.S. Pat. No. 3,870,033. Thus, UV sterilization is conventionally effected by irradiation from a low-pressure mercury lamp, i.e. by UV light mainly of 2537.ANG.. Clearly, under such conditions, there occurs no oxidation of organic impurities included in TOC content at all.
Under the state of the prior art as above-mentioned and in view of economy of resources and of prevention of environmental pollution, further development and improvement in the preparation of ultra pure water with higher efficiency and with higher quality is eagerly desired in order to solve the problems involved in the recovery and reclamation of water used more effectively and to cope with the increase in demand of ultra pure water of higher purity.